Noise filter

1. A noise filter configured to suppress a common mode voltage that is generated in cables connected to an electric power converter, comprising: detecting capacitors connected to the cables, respectively, and configured to detect the common mode voltage; an operational amplifier having a positive input terminal via which the common mode voltage detected by the detecting capacitors is inputted; an emitter follower circuit having an input terminal connected to an output terminal of the operational amplifier and having an output terminal connected to a negative input terminal of the operational amplifier; and a transformer configured to apply a compensating voltage to each of the cables by applying a voltage at the output terminal of the emitter follower circuit to each of the cables in opposite phase.

2. The noise filter according to claim 1 , wherein assuming that a is a gain of the voltage at the output terminal of the emitter follower circuit with respect to the common mode voltage that is inputted to the operational amplifier, a turns ratio of the transformer is adjusted to be α:1.

3. The noise filter according to claim 1 , further comprising: a first resistor via which the output terminal of the emitter follower circuit and the negative input terminal of the operational amplifier are connected to each other; and a second resistor via which the negative input terminal of the operational amplifier and a neutral point of a power supply that supplies electric power to the operational amplifier are connected to each other.

4. The noise filter according to claim 1 , further comprising a common mode choke coil provided on the cables.

5. The noise filter according to claim 1 , further comprising Y capacitors connecting the cables to a ground potential, respectively.

6. The noise filter according to claim 5 , wherein the Y capacitors are connected to the ground potential via a damping resistor.